Tannins, defined as water-soluble phenolic products that can precipitate proteins from aqueous solution, are naturally occurring compounds. There are two classes of tannins, the hydrolysable tannins, deriving from gallic acid and ellagic acid, and the condensed tannins, that is proanthocyanidins, which are oligomers and polymers of flavanols. Tannins inhibit the growth of a number of microorganisms and are resistant to microbial attacks (Chung, K. T., et al. (1998), Tannins and human health: A review. Critical Reviews in Food Science and Nutrition 38:421-464. Moulds and yeasts and some aerobic bacteria are usually best fitted to degrade tannins but also anaerobic degradation occurs, e.g. in the intestinal tract (Bhat, T. K., et al. (1998), Microbial degradation of tannins—A current perspective. Biodegradation 9:343-357).
Tannins are known as antinutrients, i.e. they decrease the efficiency of the body to convert digested nutrients to new body substances. However, also health beneficial effects of tannins have been reported, e.g. anticarcinogenic effects, ability to reduce blood pressure and to modulate immune-responses. These effects might be due to the antoxidative properties of tannins (Chung et al. 1998). An efficient antioxidative tannin with reported anticancerogenic properties is ellagic acid. Another type of tannin with exceptional high antioxidative capacity is proanthocyanidins, present in for example grapes and olives. Thus, tannins present in varying concentrations in plant derived foods have profound effects on human health. It is not advisable to ingest large quantities of tannins as they may be involved in cancer formation and anti-nutrition activity, but the intake of small quantity of the correct kind of tannin may be beneficial to human health by affecting the metabolic enzymes, immuno-modulation or other functions (Chung et al. 1998).
However, also the anaerobic breakdown products from many tannins, as produced in the intestinal tract, can generate compounds with health beneficial effects (Bhat et al. 1998). Such breakdown compounds are, for example, derivates of phenylpropionic or phenylacetic acids (Bhat et al. 1998). When absorbed in the GI-tract theses compounds have an anti-inflammatory effect. These compounds together with other breakdown products from tannins have also a wide range antimicrobial effect in the GI-tract, suppressing unwanted bacteria.